As an on-demand type of ink jet printing system, there are the piezoelectric system in which ink is pressurized and ejected by a piezoelectric element, and the thermal system in which ink is rapidly boiled by a heating element and the ink is ejected by generated bubbles. An example of the piezoelectric system is disclosed, for instance, in Japanese Patent Laid-Open Publication No. SHO 60-19538. FIG. 29 shows an ink jet head disclosed in the publication. In this figure, designated at the reference numeral 67 is an ink chamber, at 68 an ejection nozzle for ejecting ink therefrom, at 69 an ink supply hole, at 70 a flexible upper plate, at 71 a piezoelectric element, at 72 an ink drop, and at 73 printing paper.
Next, a description is made for operations of this printing head. An edge of the ink chamber 67 is opened via the ejection nozzle 68 for ejecting ink to atmosphere, and the other edge is communicated to the ink supply hole 69. The piezoelectric element 71 is adhered to the flexible upper plate 70 constituting a portion of a wall of the ink chamber 67. The ink is supplied through the ink supply hole 69 into the ink chamber 67. When a driving pulse is loaded to the piezoelectric element 71, the upper plate 70 is bent in the direction in which a capacity of the ink chamber 67 is reduced, and a pressure wave is generated in the chamber. Due to this pressure wave, the ink is ejected from the ejection nozzle 68, and ink drops 72 are ejected to the printing paper 73.
Conventionally, in a case where an image is printed by using this type of ink jet print-head, controls are provided only so that ink is ejected or not ejected to each pixel constituting the image. For this reason, generally in a case where gradation of an image is represented, graduation image is provided by using a digital halftoning method such as the dither method or dot pattern method. For instance, in a case of the dot pattern method, one pixel is represented with an m.times.n dot matrix, and gradation level in a range from 0 (zero) to m.times.n is expressed with a number of dots to be printed in each dot matrix, said number. For instance, when a 4.times.4 dot matrix is used, it is possible to represent 17 gradation levels from 0 to 16. For this reason, more gradation levels can be expressed by making size of a dot matrix larger. On the other hand, however, by making larger size of a dot matrix, resolution of an actual image becomes lower. In a case of a 4.times.4 dot matrix as an example, the resolution of a printed image is one fourth of the original resolution.
So as a method of representing many gradation levels without making lower resolution of a printed image, there is available such a method as that disclosed in the same publication (Japanese Patent Laid-Open Publication No. SHO 60-19538) in which a plurality types of ink each having a different density are used. Description is made for this method referring to a case where two types of ink each having a different density are used and a picture element is expressed with a 2.times.2 dot matrix. In FIG. 30, the dot pattern expressed according to the method is shown. In this figure, a dot 74 is printed with low density ink, while a dot 75 is expressed with high density ink. In a case where single density ink is used, only 5 gradations from 0 to 4 can be represented, while, when two types of ink are used, 9 gradations from 0 to 8 can be represented. Furthermore by increasing types of ink, a number of gradation levels can be increased, and it becomes possible to represent many gradation levels without making larger size of a dot matrix.
In this case, however, when one type of ink is added for use, only 4 gradations are increased, and when it is tried to realize many gradation levels, it is necessary to use many types of ink, which will result in cost increase and required a large scale system.
As a method of representing many gradation levels based on the conventional technology, also there is available a method in which a number of gradation levels are represented by changing volume of ink drops to be ejected. In this case, as a density range which can be represented with one type of ink is limited, a plurality of ink each having a different density are used, and one type of ink is selected to a density level to be reproduced. FIG. 31 shows the reflection density characteristics in a case where two types of ink each having a different density are used. In this figure, the reference numeral 76 indicates the reflection density characteristics of low density ink, while the reference numeral 77 indicates the reflection density characteristics of high density ink. This figure plotted with the horizontal axis indicating a voltage shows that, as the voltage becomes higher, the larger ink drops to be ejected and the higher reflection density can be obtained. In this method, selection of up to two types of ink and reproduction of reflection density ranges from D0 to D3 by changing volume of ink drops are possible. Herein description is made especially with reference to the reflection densities D1 and D2. To make reproduced reflection densities continuous by switching the two types of ink to be used, it is necessary to switch a voltage from the voltage ranges e2 to e3 for reflection densities D1 to D2 of the low density ink 76 to voltage range e0 to e1 for reflection densities D1 to D2 for the high density ink 77. In this step, volume of ink drops obtained in the range e3 to e3 is quite different from that obtained in the range from e0 to e1, and as a result diameters of formed dots are largely different.
In a case where a plurality types of ink each having a different density are used, there are available for the purpose to obtain the same reflection density a method in which a small dot is formed with high density ink, and a method in which a large dot is formed with low density dot, and even if the reflection densities in both cases are identical, texture of formed images is substantially different in visual feeling. Also if switching from small dots formed with high density ink to large dots formed with low density ink is executed in an area where a density level of an image signal to be printed gradually changes, continuity of the reflection densities is insured, but clear difference in visual feeling for image quality appears, and a border between dots formed with low density ink and those formed with high density ink appears as an artifact. To solve this problem, for instance, Japanese Patent Laid-Open Publication No. SHO 60-21291 discloses a method in which the problem as described above is solved by using a digital halftoning method in an area in which ink density is switched and furthermore using a combination of varied dot diameters with low density ink as well as with high density ink.
In a case where the method as described above is used, however, as a gradation expression different from a gradation to be reproduced is employed in some areas, the portion becomes all the more remarkable in some images, and an excellent image can not always be obtained. Also there is a limit for size of a dot diameter which can be realized, and in an area where a density lower than that realized with minimum dot size is required, a desired gradation can not be reproduced.
Configuration of an ink jet printer based on the conventional technology is as described above, gradation levels are expressed by means of the digital halftoning method making use of a dot matrix, and for this reason resolution of an actually printed image is inferior to that of the original image, which makes it impossible to obtain a high quality image. Also in a case where density gradation is used, discontinuous gradation representation occurs in some portions of a reproduced density area, and also in this case an excellent image quality can not be obtained. Furthermore, in an area where density is lower than a density value for minimum dot diameter size, gradation representation can not be realized, which is disadvantageous.